Planetariums are useful instruments for the study of astronomy. Clear, bright images of stars and other celestial objects may be projected onto a hemispherical screen using large fixed planetariums. These large planetariums are suitable for use in dedicated applications such as museum auditoriums but are not practical for many educational settings such as schools and universities, for personal use or for any application where modest unit cost or planetarium portability is a desirable feature.
Several smaller planetarium projection devices have been disclosed as discussed in detail hereinafter. In general, the prior art have poor image quality, e.g., typically images are dim, fuzzy or a combination of both. Typically, portable planetariums have a light source such as a halogen or tungsten filament bulb housed within the enclosure of the projector. The heat generated by the light source inside the projector can create a thermal burn hazard for planetarium operator or audience members.
One planetarium device disclosed in U.S. Pat. No. 1,693,969, employs a central arc lamp to illuminate a multitude of flat slides with tiny holes that represent stars. As that patent is understood, each slide covers a section of the sky and is imaged with a separate projection lens on the surrounding dome.
Another planetarium design, disclosed in U.S. Pat. No. 2,827,827, uses a central light source to illuminate an enclosing shape with holes or small lenses for each star. The projection mask consists of a globe having pin holes through which light was projected onto a curved screen. The light source employed was an incandescent filament bulb. The '827 design also includes the use of a spherical reflecting mirror to reduce the apparent size of the light source. Although the spherical mirror is reported to reduce the apparent point size of the light source (thereby improving the contrast of stars projected by the device), the intensity of the light source is sacrificed such that the projected stars are difficult to view except in complete darkness.
U.S. Pat. No. 4,763,280 describes an image generation system suitable for projecting a computer-generated image onto a curved surface. The system employs a projector capable of processing a computer signal into a visual output and projecting the image generated onto a screen by passing the image through a wide-angle lens.
Most existing low-cost planetariums use tungsten halogen or gas-filled tungsten filament lamps to illuminate the interior of a hemispherical enclosure having one or more apertures through which light may pass. Filament lamps have the advantage of low cost, easy replacement, small size and substantially uniform coverage over a hemisphere. These lamps have the disadvantages of filament size cannot be reduced below about 0.7 mm, low brightness, short life, high temperatures posing a burn hazard, and low color temperature. Bulb geometry may produce internal reflections and different filament profiles when viewed from different directions. Stars may appear as circles, lines or rings from different viewing angles.
Miniature arc lamps are being used increasingly in computer projectors and offer the advantages of high brightness along with small arc sizes, high color temperature, and long life. Arc intensity per unit area is much higher than a tungsten filament lamp, but arc lengths are still longer (at 1.2 mm) than miniature tungsten halogen lamps. Notable disadvantages associated with miniature arc lamps include uneven hemispherical coverage, large size, high starting voltages, and high temperature. New arc lamps are smaller than older designs, but still are too large to be used in miniature, portable projectors such as the design disclosed in the '827 patent.
It is known in the art that the quality of stars and other images projected onto a viewing surface is directly proportional to the apparent size of the light source. Typically, prior art planetarium projectors were limited by the physical size of a filament or arc of the light source.
It thus would be desirable to provide a new device which is easy to use and capable of projecting clear bright images onto a curved viewing surface. It would be particularly desirable to provide a device that would be portable by one or two individuals and capable of projecting clear bright images of stars, planets and other celestial objects onto a curved viewing screen. Such portable planetarium devices preferably would be simple in construction and less costly than prior art devices.